Add spellchecker scripts for Octave and run spellcheck of documentation interpreter/doccheck: New directory for spelling/grammar scripts. interpreter/doccheck/README: Instructions for using scripts. interpreter/doccheck/spellcheck: Script to spellcheck a Texinfo file. interpreter/doccheck/aspell.conf: GNU Aspell configuration file for Octave documentation. interpreter/doccheck/aspell-octave.en.pws: Private Aspell dictionary. interpreter/doccheck/add_to_aspell_dict: Script to add new Octave-specific words to private Aspell dictionary. interpreter/octave.texi: New @nospell macro which forces Aspell to ignore the word marked by the macro. interpreter/mk_doc_cache.m: Skip new @nospell macro when building doc_cache.

--- a/scripts/signal/stft.m
+++ b/scripts/signal/stft.m
@@ -27,9 +27,9 @@
 ## is applied:
 ##
 ## @table @asis
-## @item hanning
+## @item @nospell{hanning}
 ## w_type = 1
-## @item hamming
+## @item @nospell{hamming}
 ## w_type = 2
 ## @item rectangle
 ## w_type = 3